In view of implementation of space saving and low power consumption, demand exists to further reduce the size of gas detectors for measuring the concentration of a flammable gas or detecting leakage of a flammable gas. In recent years, gas detection elements with greatly reduced sizes have been developed by use of MEMS (Micro-Electro-Mechanical System) technology (also called the micromachining technique). A gas detection element formed by use of MEMS technology is configured such that a plurality of thin films are formed in layers on a semiconductor substrate (e.g., a silicon substrate).
Examples of such a gas detection element include a thermal-conductivity-type gas detection element and a catalytic-combustion-type gas detection element. The thermal-conductivity-type gas detection element has a heat-generating resistor and utilizes the phenomenon that, when the heat-generating resistor is energized and generates heat, heat is conducted to a flammable gas. Specifically, in the case of controlling the gas detection element at a constant temperature, conduction of heat causes a change in temperature of the heat-generating resistor and thus a change in resistance of the heat-generating resistor. On the basis of the amount of the change, a gas-to-be-detected is detected. The catalytic-combustion-type gas detection element has a heat-generating resistor and a catalyst, which causes combustion of a flammable gas by means of heat of the heat-generating resistor. The catalytic-combustion-type gas detection element utilizes the phenomenon that, when the heat-generating resistor is energized, the catalyst causes combustion of a flammable gas. Specifically, the heat-generating resistor changes in temperature and resistance according to heat of combustion of a flammable gas. On the basis of the amount of the change, a flammable gas is detected.
In both the thermal-conductivity-type gas detection element and the catalytic-combustion-type gas detection element, the resistance of the heat-generating resistor varies with the type or concentration of a flammable gas. Thus, a gas detector having such a gas detection element can detect a flammable gas on the basis of the resistance of the heat-generating resistor.
Such a gas detection element is configured as follows: an insulation layer is disposed on a semiconductor substrate, and a heat-generating resistor is disposed in the insulation layer. Preferably, the outermost surface (specifically, a surface which comes into contact with a gaseous atmosphere that contains a flammable gas) of the insulation layer has excellent corrosion resistance and excellent stability. A gas detection element fabricated by use of MEMS technology may be configured such that the outermost surface of the insulation layer is of silicon nitride (refer to Japanese Patent Application Laid-Open (kokai) No. 2005-156364). However, silicon nitride or a like material may tend to be eroded by an alkali substance adhering thereto. Thus, improvement in durability against alkali is desired.
In order to prevent erosion caused by adhesion of an alkali substance, provision of a protection layer resistant to alkali (hereinafter, referred to as the alkali-resistant protection layer) on the surface formed of silicon nitride or the like is conceived (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2005-164570). According to Japanese Patent Application Laid-Open (kokai) No. 2005-164570, the alkali-resistant protection layer is formed by a so-called spin coating process. Specifically, alumina sol is applied to the surface in a layered manner, followed by firing. By this process, an alumina layer (i.e., an alkali-resistant protection layer) is formed.
There has also been proposed a configuration in which the outermost surface layer of a gas detection element is formed of an oxide film exhibiting high alkali resistance (Japanese Patent Application Laid-Open (kokai) No. 2010-096727). This configuration can prevent erosion of the gas detection element even when an alkaline substance adheres to the surface of the element.
The oxide film formed on the gas detection element described in Japanese Patent Application Laid-Open (kokai) No. 2010-096727 has gas impermeability (i.e., a dense structure). Thus, impurities (e.g., an organic silicon compound) can be prevented from entering the oxide film.
That is, this configuration can prevent impurities from entering the outermost surface layer of the gas detection element, to thereby reduce a change in thermal capacity. Accordingly, the output of the gas detection element is stabilized and becomes accurate, whereby high detection accuracy can be achieved.